1. Field of the Invention
The present invention generally relates to frequency modulation methods and modem units, and more particularly to a frequency modulation method which carries out a frequency modulation using a common filter with respect to a mode which uses a large number of carrier frequencies, and to a modem unit which employs such a frequency modulation method.
2. Description of the Prior Art
Conventionally, various kinds of modem units which employ the frequency modulation technique have been proposed. When using a plurality of carrier frequencies, a modem unit on the transmitting end must be provided with a filter for each of the carrier frequencies. On the other hand, a modem unit on the receiving end must be provided with bandpass filters having passbands for passing the frequencies corresponding to the original binary data values. The original binary data values are judged depending on the existence of outputs of the bandpass filters.
However, when a number of filters corresponding to the plurality of carrier frequencies are provided, the load on the modem design increases, and there were problems in that the construction of the modem unit becomes complex and the modem unit cannot be manufactured at low cost.
On the other hand, in order to judge the data by a digital signal processing by subjecting the received data to an analog-to-digital conversion, it is necessary to use a bandpass filter having a narrow band. But in order to realize the bandpass filter having the narrow band, it is inevitably necessary to set the number of taps of the bandpass filter to a large value, so as to satisfy a sufficient blocking characteristic. As a result, there were also problems in that the amount of computation processes and delays in the bandpass filter become large, and that it is difficult to improve the performance of the modem unit.